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Generally 0.1 μF 50 V ceramic capacitors are connected across the motor terminals to reduce noise. How this value is decided? Does it depend on the motor rating?

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100nF was and still is a very common value at 50VDC . The precise value is not important so ceremic capacitors are used . They are cheaper and have lower ESR so they actually work better than the physicaly larger more expensive metal film caps .These caps were decided on before larger ceremic caps in the microfarad range became cheap.Larger caps will have a lower self resonant frequency especially when considering the practical lead lengths in a motor.This could make big caps useless at suppressing radiated EMC.So using huge caps on a huge motor is not the answer . Feedthrus are often used for high currents.Most orthodox PWM controllers wont like a large cap across the motor terminals because the mosfet would be turning on into it .

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